The present CCD (charge coupled device) imager fabrication technology is such that large area CCD arrays suitable for use in broadcast quality television cameras suffer from a very low yield. The low yield is a direct result of imperfections and defects in the manufactured CCD semiconductor chip, which yield in some cases may decrease at a rate related to the fourth power of the area of the chip. This results in a relatively high cost and reduced availability of CCD imagers suitable for high quality television cameras. These defects may manifest themselves as single picture element (pixel) white or black spots of various amplitude in the reproduced image.
The use of defect correction apparatus in conjunction with solid-state imagers having such defects is highly advantageous, in that a greater number of solid-state imagers may become usable, thereby improving the yield of usable imagers and lowering their cost. It is well known that a single pixel defect may be removed from the video signal representative of an image by substitution of picture information from surrounding pixels. For example, U.S. Pat. No. 3,904,818 issued Sept. 9, 1975 in the name of Kovac and assigned like the present application to RCA Corporation shows a system which detects when a photosensor provides excessive dark current and, in response, substitutes a replacement signal that is the average of signals provided by photosensors surrounding the defective one.
In some applications, such as imagers intended for use in broadcast television cameras, signal averaging type defect correctors may not be considered acceptable, since signal averaging, by its nature, introduces artifacts into the television picture. For example, when the camera is viewing a scene having fine detail in the area where the pixel defect is located, the defect corrected signal most probably will not accurately represent the lost picture detail due to the signal averaging.
U.S. Pat. No. 4,481,539 issued Nov. 6, 1984 in the name of Meise et al. and assigned like the present application to RCA Corporation, discloses a defect correction arrangement which does not use signal averaging techniques and instead simultaneously processes corresponding signals derived from two imagers registered to view the same image. The signals provided from the imagers are combined except when one is providing a signal from a defective pixel. In that case, only the signal from the good pixel is used. Although this technique does not generate the aforenoted undesirable signal averaging artifacts, the use of two imagers is required which necessarily increases the cost and size of the defect correction apparatus. Consequently, it is desirable to provide defect correction apparatus which does not require a second imager, i.e., one which develops a replacement signal for a defective pixel by substitution of a signal derived from at least one nearby good pixel.
It has been recognized by the present inventor that many of the imagers rejected during manufacture due to defective pixels in fact contain mostly low contrast single pixel defects. Low contrast defects are those which are not visible in a displayed image under normal lighting conditions due to their relatively low amplitude and only become objectionably noticeable during low light level or high temperature operating conditions. If these imagers were usable for broadcast quality imaging, the yield of usable broadcast quality imagers could be significantly increased, thereby reducing the cost per imager.